1. Field of the Invention
The invention concerns pyrogenic, oxidic particles containing superparamagnetic metal oxide domains in a non-magnetic metal oxide or metalloid oxide matrix, processes for their production and their use.
2. Discussion of the Background
Superparamagnetic particles are used in many areas, for example for data stores, as contrast media in imaging processes, in ferro fluids or for biochemical separation and analysis processes.
Superparamagnetic materials have properties that are characteristic for both paramagnetic substances and ferromagnetic materials. Like paramagnetic substances, superparamagnetic substances have no permanent (equiaxed) alignment of the elementary magnetic dipoles in the absence of external magnetic fields. On the other hand they display a similarly high magnetic susceptibility under the influence of an external magnetic field. Furthermore they are characterised by the presence of crystalline structures. Superparamagnetism occurs when the diameter of the crystalline regions in a normally ferromagnetic substance falls below a particular critical value.
The theoretical basis of superparamagnetism lies in the thermal destabilisation of the permanent alignment of the elementary magnetic dipoles in the crystal structure. The thermal energy of the elementary magnetic dipoles inhibits their alignment in the absence of external magnetic fields. Following the removal of an external magnetic field, the individual elementary magnetic dipoles are still present, but they are in such a thermally excited state that they cannot align themselves in a parallel (equiaxed) orientation. Correspondingly the crystals are not permanently magnetic.
Typical superparamagnetic substances are maghemite (gamma-Fe2O3, xcex3-Fe2O3) and magnetite (Fe3O4), which display superparamagnetic behaviour below a particle size of approx. 20 nm, depending on the substance and shape.
The superparamagnetic properties of such particles are only retained if the magnetic domains are physically separated. To this end the particles are typically coated with organic compounds and stabilised to prevent aggregation.
Superparamagnetic iron oxide particles for example can be obtained by spray pyrolysis of the iron compounds iron(III) acetyl acetonate, iron(II) ammonium citrate and iron(III) nitrate (T. Gonzxc3xa1les-Carrexc3x1o et al., Materials Letter 18 (1993) 151-155) or by a gas phase reaction starting from iron pentacarbonyl or iron acetyl acetonate (S. Barth et al., Journal of Material Science 32 (1997) 1083-1092).
The disadvantage of spray pyrolysis is that the choice of starting materials and of reaction conditions that lead to gamma iron oxide (xcex3-Fe2O3) is limited. If iron(III) chloride is used, ferrimagnetic particles are obtained. Furthermore, alpha iron oxide (xcex1-Fe2O3) and hydroxide phases often appear as impurities.
The choice of iron oxide precursors is equally restricted in the gas phase reaction. Starting materials containing chlorine, sulfur or nitrogen are explicitly excluded since they can lead to the formation of undesirable iron oxide phases, such as e.g. beta iron oxide (xcex2-Fe2O3) if iron chloride is used as precursor.
U.S. Pat. No. 5,316,699 describes the production of ultrafine superparamagnetic particles in a dielectric matrix by means of a sol-gel process and the subsequent reducing treatment with hydrogen. The particles obtained display a network of interconnected pores, in which the magnetic component is located. The disadvantage compared with largely pore-free particles of the same surface area is that in applications involving mass transport processes the pores may not be freely accessible.
Also disadvantageous is the lengthy production of the particles, which can last for up to several weeks, and the necessary aftertreatment with hydrogen at uneconomically high temperatures. In addition the particles can contain impurities from the starting materials along with by-products and decomposition products from the other reaction steps.
Zachariah et al. (Nanostruct. Mater. 5, 383, 1995) describe nano-materials consisting of silicon dioxide and superparamagnetic domains from iron oxides obtained by flame oxidation. They start from organic precursors, toxic iron pentacarbonyl and hexamethyl disiloxane. These materials are uneconomic for producing relatively large quantities and there is also the risk that carbon impurities may remain in the particles. Furthermore, only particles with silicon dioxide as non-magnetic component and iron oxides as superparamagnetic component are described.
A further characteristic of superparamagnetic particles is their xe2x80x9cblocking temperaturexe2x80x9d. This is the temperature below which any superparamagnetic behaviour ceases to be observed. In particles obtained by the process described, it is 155 K. A further reduction in the xe2x80x9cblocking temperaturexe2x80x9d for special applications, for example in cryogenic engineering, is desirable.
The object was therefore to provide superparamagnetic particles that do not display the disadvantages of the prior art. In particular they should be largely free from impurities, such as carbon and non-superparamagnetic modifications, for example, and display only a low pore volume.
The object of the invention is further to provide a process with which a broad range of superparamagnetic particles can be produced from readily available, inexpensive starting materials.
The invention provides pyrogenic, oxidic particles with a chloride content of 50 to 1000 ppm, containing superparamagnetic metal oxide domains with a diameter of 3 to 20 nm in a non-magnetic matrix containing a metal oxide or a metalloid oxide. The particles can be produced by pyrogenic processes by mixing the precursor of the superparamagnetic domains and the precursor of the non-magnetic metal or metalloid oxide matrix in a flame with air and/or oxygen and fuel gas and reacting this mixture in a flame.